1. Field of the Invention
On the industrial scale, cement is made through mixing and grinding of clinker and gypsum with corrective substances like lime, slag and pozzolana. In the process for producing cement according to what is known as “dry” technology, the clinker is obtained by high temperature baking of a mixture of raw materials consisting mainly of lime (calcium carbonate) and clay (silica, alumina, iron oxides, as well as crystallisation water). The raw materials are mixed in solid state in the desired proportions and then finely ground until a homogeneous powder known as “raw meal” is obtained. In the present description, by “raw meal” we thus mean the homogeneous dust thus obtained used as starting material for producing clinkers.
The raw meal is transformed into a clinker by means of baking at a temperature of about 1450° C. in a rotary kiln essentially consisting of an inclined rotary cylinder.
During its transit in the rotary kiln, the raw meal is heated up to temperatures of about 1450° C. During heating the meal firstly undergoes complete calcining and, thereafter, reacts forming the calcium silicates and aluminates (clinkering reaction) that represent the main constituents of the clinker. More specifically, during the clinkering reaction there are a series of chemical reactions between calcium oxide, silicon oxide, aluminium oxide and iron oxide, said reactions being encouraged by the melting of a part of the raw materials themselves (aluminium and iron oxides).
The energy necessary to make the clinkering reaction take place is produced by means of a burner positioned on the head of the rotary kiln, at the opposite end with respect to that in which the meal is loaded. The fuels generally used are coal, petcoke, fuel oil, methane, as well as alternative fuels like, for example, meat meals.
The heat energy is transmitted to the raw meal subjected to treatment by irradiation in the baking area at the burner (temperature of about 2000° C.) and by convection and conduction by means of the combustion gases in the remaining part of the kiln.
At the end of the baking treatment, the clinker thus obtained is discharged from the rotary kiln and is quickly cooled in an air cooler in order to stabilise it.
2. Description of Related Art
The processes according to the state of the art are represented and discussed with reference to the following figures:
FIG. 1A, which shows a schematic representation of a clinker production plant according to the state of the art comprising a rotary kiln equipped with a 4 stage suspension preheater;
FIG. 1B, which shows a schematic representation of a clinker production plant according to the state of the art comprising a rotary kiln equipped with a 5 stage suspension preheater and a precalciner.
In the aforementioned figures the full lines indicate the flows of solid material, the dashed lines indicate the gaseous stream flows, whereas the Roman numerals indicate the stages of the suspension preheaters.
In clinker production plants known in the state of the art, the raw meal, before being fed to the rotary kiln, is subjected to a preheating and, optionally, precalcining treatment.
One of the currently most widely used preheating techniques is based on the use of the so-called “suspension preheater” or “multi-stage cyclone preheater” (hereafter also just “preheater”), consisting of a cyclone tower in which each preheating stage takes place in one or more cyclones. In such a type of preheater, by first cyclone we mean the cyclone in which the first preheating stage and the first separation between preheated meal and combustion fumes take place, by second cyclone we mean the cyclone in which the second preheating stage and the second separation between preheated meal and combustion fumes take place and the subsequent cyclones of the multi-stage cyclone preheater are analogously defined. In the present description, the first cyclone of the preheater, just like the subsequent cyclones, should always be interpreted according to the above definition.
The first stage, unlike the subsequent ones, is configured to minimize the carrying of dust by the combustion fumes from the kiln. Despite this, the concentration of dust in the combustion fumes exiting the preheater stays high (around 50-100 g/Nm3).
The preheating and precalcining steps are carried out, respectively, in the preheater 1 and in the precalciner 2 (FIGS. 1A and 1B). The presence of these steps allows the partially calcined (30-40%) meal that has been preheated to a temperature of about 950° C. to be fed to the rotary kiln 3, with a substantial energy saving in the subsequent clinkering reaction. The presence of the preheating step, optionally accompanied by the precalcining step, also allows rotary kilns of reduced size to be used, thus reducing the heat losses that occur in such kilns and increasing the overall energy efficiency of the clinker production process.
In the preheater, the starting raw meal is gradually brought from the temperature of about 40° C. to about 950° C. The heating is carried out keeping the meal in suspension in a flow of hot gases, consisting of the combustion fumes of the rotary kiln, exploiting the large heat exchange surface between the meal and the combustion fumes.
In the preheating step the amount of time for which the solid phase (meal) is in contact with the gaseous phase (combustion fumes of the rotary kiln) is of fundamental importance. In order to ensure an optimal contact time between the solid phase and the gaseous phase, the suspension preheater consists of a series of cyclones (from 4 to 6) arranged one on top of the other to form a tower of variable height even up to 130-150 m. Such a preheater can be defined as a multi-stage cyclone preheater. The first preheating stage, which occurs at the top of the tower, can be carried out in two cyclones in parallel to ensure better efficiency of separation of the meal from the gaseous flow before it exits the preheater.
With reference to FIG. 1A, in the multi-stage cyclone preheater 1 the combustion fumes from the rotary kiln 3 and having a temperature of about 900-1000° C. pass through the cyclones from the bottom towards the top (from IV to I). The starting raw meal is mixed with the combustion fumes in the preheater 1, inside which it is inserted through an inlet 4, arranged at the top of the preheater, between the first (I) cyclone and the second (II) cyclone. The raw meal passes through the preheater up to the outlet in the lower part, transported from one cyclone to the next by the flow of combustion fumes. In each cyclone about 80% of the solid phase (meal) is separated from the gaseous phase (combustion fumes) to then be inserted once again in the gaseous phase entering into the cyclone below. The gaseous phase containing the remaining solid fraction (about 20% of the meal), on the other hand, flows to the next cyclone above.
At the bottom of the preheater 1, a preheated meal is obtained having a temperature of about 950° C. From the last preheating stage in the multi-stage cyclone preheater, the meal is discharged directly into the rotary kiln 3 for the subsequent clinkering reaction.
In plants equipped with a precalciner 2 (FIG. 1B), the preheated meal is fed from the preheater 1 to a suitable combustion chamber 5, equipped with a burner 6, inside which it undergoes a partial calcining process. The precalcined meal leaves the precalciner 2 and is fed, together with the combustion fumes of the precalciner 2, to the last stage (V) of the preheater 1 to then proceed towards the rotary kiln 3. The combustion fumes of the precalciner 2 flow together with those of the rotary kiln 3 and climb the preheater 1 up to the top outlet 7, after the first cyclone.
The gaseous flow exiting through the outlet 7 of the preheater, comprising the combustion fumes of the rotary kiln 3 and, optionally, those of the precalciner 2, has a temperature of about 270-360° C. Before being released into the atmosphere, this flow is generally used in other steps of the cement production process (for example, for grinding and drying the raw materials or else as combustion air in the rotary kiln or in the precalciner) to recover the heat content.
The preparation of the clinker in a cement production plant like the one described above generates enormous volumes of gaseous emissions, which can potentially pollute the environment.
The gaseous flow exiting the preheater is characterised by a high concentration of polluting substances, in particular nitrogen oxides (NO.) and dusts.
The NOx derive mainly from the combustion processes that take place in the rotary kiln and, optionally, in the precalciner. The main techniques currently used to reduce the NOx in the gaseous flow exiting the preheater are the following two:                Selective Non-Catalytic Reduction (SNCR) that foresees the reaction of the NOx with a reducing agent (for example ammonia or urea) in the high temperature area of the preheater;        Selective Catalytic Reduction (SCR) that foresees the reaction of the NOx with NH3 as reducing agent in the presence of a catalyst.        
The SNCR technique is effective if used on a gaseous flow having a temperature of 800-900° C. and allows most of the NOx present to be reduced (i.e. more than 50%).
The application of the SCR technique, only recently used in the field of electrical energy production and in the development phase in the field of cement, allows very high reduction yields (over 90%) to be achieved. The SCR technique is effective if used on a gaseous flow having a temperature of between about 300 and 400° C.
Considering this optimal temperature range, the SCR reduction system is installed in clinker production plants at the top outlet of the preheater, after the first cyclone, where the gaseous flow exiting through such an outlet, comprising the combustion fumes of the rotary kiln and, optionally, those of the precalciner, has a temperature of about 270-360° C.
The application of SCR technology does, however, have various drawbacks due mainly to the presence of substantial quantities of dust in the combustion fumes exiting the preheater. The dust, depositing on the walls of the catalyst, reduce the efficiency of the SCR reduction system, at the same time increasing the resistance to the passage of the gaseous flow and therefore the energy consumption linked to moving it.
The presence of dust in the treated gaseous effluent also means high energy consumption associated with the need to clean the catalyst with compressed air, as well as reducing the useful life of the catalyst due to the abrasive action that the dust exerts on the surface of the catalytic bed.
The high presence of dust is linked essentially to the limited efficiency of dust removal of the cyclones that make up the preheater. Although they are designed to maximise the separation efficiency, the cyclones are only able to effectively separate heavier dust.
Secondly, at the outlet of the preheater there can also be ashes generated by the combustion in the burners of the rotary kiln and of the precalciner of alternative fuels, like for example animal meals. The presence of ashes (containing phosphates) causes the catalyst to be poisoned and its NOx reduction effectiveness to consequently be decreased.
Sometimes in the fumes there are sulphur oxides, mainly in the form of SO2, depending on the sulphur content of the raw materials used.
The SO2 reduction in these cases can be carried out by means of injection of calcium oxide- and/or calcium hydroxide-based compounds in the combustion fumes, with consequent formation of calcium sulphate, said calcium sulphate advantageously being able to be recycled in the clinker production process. The effectiveness of reduction of the sulphur oxides in gaseous phase according to the aforementioned technique is also limited by the presence in the fumes of high concentrations of dust, which make it almost impossible to recycle unreacted lime.
The combustion fumes exiting the preheater, after having been purified of NOx and SOx and after having optionally been recycled through other steps of the production process to recover its residual heat, must finally be removed of dust before being released into the atmosphere.
The dust-removal process is normally carried out through filtering with electrofilters (also known as electrostatic precipitators) or else with fabric filters, the latter being most widely used in clinker production plants.
Unlike electrofilters that can filter gaseous flows even at a high temperature, fabric filters can only operate at temperatures below 250° C. (according to the type of fabric used). Therefore, the use of fabric filters to remove dust from the combustion fumes necessitates the installation of suitable systems for reducing the temperature of the gases to be filtered (for example, conditioning towers, heat exchangers, diluting air insertion) with a consequent increase in the investment costs and the energy consumption of the process.
The preparation of clinker in a cement production plant like the one described above also has other drawbacks.
The short contact time between the solid phase (meal) and the gaseous phase (combustion fumes) that can be made in a single stage of the suspension preheater necessitates the installation of many cyclones, making a multi-stage preheater, to reach an acceptable heat exchange level. This requires the construction of strong support structures of the preheater and, at the same time, high energy consumption associated with the transportation of the fumes and of the solid material through the preheater.
According to the production capacity of the plant and the specific production technologies adopted, the preheater can reach considerable heights (130-150 m), also causing “landscape defacement”.
Moreover, even using 4-6 stage preheaters that allow extended contact times, the heat exchange between meal and combustion fumes is not sufficiently high to ensure that thermal equilibrium is reached. In preheaters known in the state of the art, the difference between the temperature of the meal and that of the combustion fumes exiting the preheater is on average about 40-50° C.